The concept of earliest known use of a particular postmark - not a stamp - is one of interest to a very limited number of people. As I advised you on your other thread, £2-3 would be a good price for these.
The database posted in your other thread is rather incomplete. the earliest hit for any Kilburn cancellation is 8 November 1877. The duplex canceller was introduced to London district offices in 1854. The circled number 60, according to the GB Philatelic Society website was already listed in 1874. It is unlikely this was any other than a duplex cancellation.
There is a very good chance that an older example will pop up sooner rather than later.